Technical Field
The present disclosure relates to an imaging apparatus that is mountable on a digital still camera, a digital video camera, and a mobile phone and performs focus adjustment using an image signal acquired by an image sensor that performs photoelectric conversion on a subject image formed by an image optical system.
Description of the Related Art
In digital cameras and video cameras, an autofocus (hereinafter referred to as AF) scheme that achieves focus by using output signals from an image sensor, such as a CCD and a CMOS, and detecting a signal corresponding to contrast of a subject, or so-called contrast detection AF, is common.
Some proposed conventional types of evaluation value for the contrast detection AF include a line-peak integral evaluation value obtained by integrating in a vertical direction a peak value of the evaluation value of each horizontal line. This type of evaluation value is stable due to the effect of the integration and is unsusceptible to noise. Hence, this is suitable for in-focus position detection and direction determination in a case where signals sensitively change in response to a slight focus shift (Japanese Patent Application Laid-Open No. 07-298120 A).
Furthermore, a possible solution that has been proposed to a problem with the contrast detection AF in that focus may not be achieved for a high luminance subject is to detect a relative minimum of a line-peak integral evaluation value to perform focus adjustment (Japanese Patent Application Laid-Open No. 2006-189634 A). With the line-peak integral evaluation value described above, a lens position at which the value is increased is not necessarily an in-focus point in some cases. FIG. 9 is a diagram of relationship between a line-peak integral evaluation value and a focus lens position for a normal subject.
FIG. 10 is a diagram of relationship between a line-peak integral evaluation value and a focus lens position for a point light source subject, such as a night scene. For the normal subject, the focus lens position at which the line-peak integral evaluation value reaches its maximum is an in-focus point. For the point light source subject, such as a night scene, the focus lens position at which the line-peak integral evaluation value reaches its maximum is not an in-focus point. This is because the point light source in an image changes in size with a degree of focusing. This will now be described in detail with reference to FIGS. 11A and 11B.
FIGS. 11A and 11B are diagrams of example images in focus and not in focus of a point light source subject, such as a night scene. As illustrated in FIG. 11A, a peak value of each line is large but the number of lines on which the subject image lies is small in the in-focus image. As illustrated in FIG. 11B, the point light source in the image is blurred to grow in size in the not-in-focus image.
Thus, the peak value of each line is small, but the number of lines on which the subject image lies is larger, resulting in an increased line-peak integral evaluation value.
As described above, in the focus adjustment performed in such a manner that the line-peak integral evaluation value is increased, focus may not be achieved for a point light source subject.
A possible solution that has been proposed to such a problem is, as described above, to detect a relative minimum of the line-peak integral evaluation value to perform the focus adjustment.
There has been a problem, however, with a screen in which a high luminance portion is present in that a focus lens position at which the line-peak integral evaluation value reaches its relative minimum is not necessarily an in-focus position. FIG. 12 is a diagram for describing a case where a focus lens position at which a line peak reaches its relative minimum is not an in-focus position. A horizontal axis represents a focus lens position, and a vertical axis represents a line-peak integral evaluation value.
In FIG. 12, the line-peak integral evaluation value lacks change in proximity to the in-focus position. This is because the form of the line-peak integral evaluation value in proximity to the in-focus position varies with conditions of the point light source, such as its size and the number thereof, within a range (an AF evaluation range) for which the line-peak integral evaluation value is calculated, and a spatial frequency band extracted for the calculation of the line-peak integral evaluation value. In general, the higher the frequency band is for the calculation of the evaluation value, the likelier it becomes that the line-peak integral evaluation value is formed to have a projection in proximity to the in-focus position, whereas the lower the frequency band, the likelier it becomes that the line-peak integral evaluation value is formed to have a depression in proximity to the in-focus position.
Thus, the line-peak integral evaluation value of a medium frequency band may lack change as illustrated in FIG. 12. If the amount of change in line-peak integral evaluation value in proximity to the in-focus position is small as illustrated FIG. 12, a relative maximum and a relative minimum of the line-peak integral evaluation value may not be detected, resulting in degradation in accuracy of in-focus position detection.